1. Field of the Invention
The present invention relates generally to methods of predicting subterranean rock properties from reflection seismic data. In another aspect, the invention concerns a system for predicting lithology and pore fluid of a subterranean formation based on reflection seismic data and well log data.
2. Description of the Prior Art
The use of seismic surveys is now fundamental in the search for oil and gas reservoirs in the earth. Seismic surveys are typically performed by imparting acoustic energy of a known amplitude and frequency pattern at one or more locations of the earth (either at a land surface or in a marine environment) and then detecting reflected and refracted acoustic energy at other locations. The time delay between the imparting of the acoustic energy at the source location and the detection of the same wave at a receiver location is indicative of the depth at which a particular reflecting geological interface is located. The field of seismic data interpretation is concerned with techniques for analyzing the detected seismic waves to determine both the location and the properties of various geological strata.
While reflection seismic data has been very successful in determining the location of certain geological strata, changes in reservoir petrophysical properties (i.e., rock properties) are typically difficult to determine from the reflection seismic data due to the limited amount of information that these properties provide in the reflected signal. However, amplitude-versus-offset (AVO) analysis is a well-known technique that can be used to make predictions of petrophysical properties from reflection seismic data. According to one aspect of the AVO approach, subsurface petrophysical properties can be estimated by calculating the zero-offset amplitude (A) of reflected seismic energy and the gradient (B) (i.e., rate of change in amplitude with offset) of the reflected seismic energy. The product A*B is thought to give an indication of the presence of certain hydrocarbon-bearing sand formations. Further, the location of a reflection seismic data point plotted on an A-B graph (with zero-offset amplitude as the x-axis and gradient as the y-axis) can provide an indication of subsurface petrophysical properties based on the location of the plotted reflection seismic data point relative to a known background trend. Although such AVO analysis can be helpful in estimating petrophysical properties in certain instances, the predictions from conventional AVO analysis are many times inaccurate and/or incomplete. Accordingly, using reflection seismic data alone, it is difficult to identify rock properties that provide an indication of successful oil and gas production from a reservoir.
Various methods of employing well log data to complement and improve the accuracy of predictions made from reflection seismic data are well known in the art. Such methods generally involve obtaining depth-scale well logs which include data such as lithologic composition, porosity, density, and liquid saturation. From these logs, synthetic time-scale seismic traces can be computed. These synthetic traces are useful for demonstrating to geophysicists what a reflection seismic time-scale trace should look like in the presence of the geologic conditions at or near the subsurface location from which the well log was obtained.
Most traditional techniques for matching reflection seismic data to synthetic data generated from well logs provide only a single predicted answer for the petrophysical properties at any given location. Thus, conventional seismic inversion schemes do not provide results that can be statistically analyzed for purposes of risk evaluation. Further, some conventional seismic inversion techniques require mathematically complex techniques for matching reflection seismic traces with synthetic seismic traces. Such complex computational processes can require expensive high-end computers and can take long periods of time to perform.
It is, therefore, an object of the present invention to provide a method for more accurately predicting rock properties of a subsurface formation from reflection seismic data and well log data.
Another object of the invention is to provide a method for predicting rock properties of a subsurface formation using seismic attributes that can readily be determined from conventional P-wave seismic surveys and that are most predictive of formation petrophysical properties.
Still another object of the present invention is to provide a method for predicting rock properties of a subsurface formation that provides results which can be statistically analyzed for purposes of risk evaluation.
A still further object of the invention is to provide a more efficient method of inverting reflection seismic data using synthetic seismic data that is mathematically simple and does not require long periods of time and/or high-powered computer systems to perform.
It should be understood that the above-listed objects need not all be accomplished by the invention claimed herein. Further objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment, the claims, and the drawing figures.
Accordingly, in one embodiment of the present invention, there is provided a computer-implemented method of predicting rock properties of a subsurface formation using reflection seismic data and well log data. The method comprises the steps of: (a) generating synthetic seismic data from the well log data; (b) quantizing the synthetic seismic data into discrete synthetic data subcells based on seismic attributes of the synthetic seismic data, with the seismic attributes (e.g., near trace amplitude, gradient, isochron, sonic, and phase angle) being predictive of rock properties; (c) quantizing the reflection seismic data into discrete reflection data subcells based on the same seismic attributes that were used to quantize the synthetic seismic data; (d) correlating each reflection data subcell with a corresponding synthetic data subcell based on the seismic attributes of the reflection and synthetic seismic data; and (e) assigning a statistical distribution of well log rock properties associated with the synthetic seismic data subcells to corresponding reflection data subcells.
In accordance with another embodiment of the present invention, there is provided a computer-implemented method of predicting rock properties of a subterranean formation using reflection seismic data and well log data. The method comprises the steps of: (a) generating synthetic seismic data from the well log data; (b) calculating synthetic seismic attributes from the synthetic seismic data, with the synthetic seismic attributes including synthetic amplitude, gradient, and at least one additional synthetic attribute selected from the group consisting of synthetic phase angle, isochron, and sonic; (c) calculating reflection seismic attributes from the reflection seismic data, with the reflection seismic attributes including reflection amplitude, gradient, and at least one additional reflection attribute selected from the group consisting of reflection phase angle, isochron, and sonic; (d) quantizing the synthetic and reflection seismic data into an equal number of synthetic and reflection data subcells so that each reflection data subcell corresponds to one synthetic data subcell based on the values of the synthetic and reflection seismic attributes; and (e) inverting at least a portion of the reflection seismic data by assigning well log data associated with the synthetic data subcells to reflection seismic data points landing in corresponding reflection data subcells.
In accordance with a further embodiment of the present invention, there is provided a computer-implemented method of predicting lithology and pore fluid of a subterranean formation using reflection seismic data and well log data. The reflection seismic data represents a surveyed volume of the formation. The well log data is either actual well log data taken within the surveyed volume or hypothetical well log data estimating expected rock properties of the surveyed formation. The method comprises the steps of: (a) generating synthetic seismic data from the well log data; (b) calculating synthetic amplitude and gradient values from the synthetic seismic data; (c) quantizing the synthetic seismic data into synthetic data subcells via partitioning of the synthetic amplitude and gradient values into user-defined numbers of amplitude and gradient partitions; (d) assigning the well log data associated with the synthetic seismic data to the synthetic data cells so that a plurality of synthetic data subcells have a statistical distribution of the well log data associated therewith; (e) observing the statistical distribution of the well log data for at least one synthetic data subcell to check the appropriateness of the number of amplitude and gradient partitions; (f) calculating reflection amplitude and gradient values from the reflection seismic data; (g) quantizing the reflection seismic data into reflection data subcells via partitioning of the reflection amplitude and gradient values into the same number of amplitude and gradient partitions as the synthetic amplitude and gradient values respectively; (h) defining amplitude and gradient scalars for approximately matching the reflection amplitude and gradient with the synthetic amplitude and gradient respectively; (i) comparing the fit between the synthetic amplitude and gradient and the reflection amplitude and gradient to check the appropriateness of the amplitude and gradient scalars; and A) inverting the reflection seismic data by assigning the statistical distribution of the well log data from the synthetic data subcells to reflection seismic data points landing in the corresponding reflection data subcells.
In accordance with still another embodiment of the present invention, there is provided a programmed storage device readable by computer. The device tangibly embodies a program of instructions executable by the computer for predicting rock properties of a subsurface formation using reflection seismic data and well log data. The program of instruction comprises the steps of: (a) generating synthetic seismic data from the well log data; (b) quantizing the synthetic seismic data into discrete synthetic data subcells based on seismic attributes of the synthetic seismic data, with the seismic attributes being predictive of rock properties; (c) quantizing the reflection seismic data into discrete reflection data subcells based on the same seismic attributes that were used to quantize the synthetic seismic data; (d) correlating each reflection data subcell with a corresponding synthetic data subcell based on the seismic attributes of the reflection and synthetic seismic data; and (e) assigning a statistical distribution of well log rock properties associated with the synthetic seismic data subcells to corresponding reflection data subcells.
In accordance with a still further embodiment of the present invention, there is provided an apparatus for predicting rock properties of a subsurface formation using reflection seismic data and well log data. The apparatus comprises a computer programmed to carry out the following method steps: (a) generating synthetic seismic data from the well log data; (b) quantizing the synthetic seismic data into discrete synthetic data subcells based on seismic attributes of the synthetic seismic data, with the seismic attributes being predictive of rock properties; (c) quantizing the reflection seismic data into discrete reflection data subcells based on the same seismic attributes that were used to quantize the synthetic seismic data; (d) correlating each reflection data subcell with a corresponding synthetic data subcell based on the seismic attributes of the reflection and synthetic seismic data; and (e) assigning a statistical distribution of well log rock properties associated with the synthetic seismic data subcells to corresponding reflection data subcells.